Honeycomb structural body

ABSTRACT

Disclosed is a honeycomb structure  10  made by joining a plurality of honeycomb segments  1, 2  in each of which a plurality of cells each having a rectangular sectional shape are formed between partition walls  3  which are at right angles to each other. At least a portion of the honeycomb segments  1  constituting the outer periphery  5  of the honeycomb structure  10  has a structure in which compression strength is larger than that of the honeycomb segments  2  constituting the other portions of the honeycomb structure  10.

TECHNICAL FIELD

The present invention relates to a honeycomb structure. Morespecifically, the present invention relates to a honeycomb structurecomposed of a combination of a plurality of honeycomb segments.

BACKGROUND ART

Ceramic honeycomb structures are actively used in various applicationssuch as a catalyst carrier applied to combustion equipment such as aboiler and the like, internal combustion engines such as a diesel engineand the like, a chemical reactor, a reformer for fuel cell and the like,or a filter for trapping and removing particulate matters contained inexhaust gases, by making use of the characteristics of ceramic qualityexcellent in heat resistance.

Although these honeycomb structures are installed in a case with theouter peripheral side thereof gripped, they are often installed in sucha manner that the outer peripheral side thereof is gripped with a largeforce as in a case that they are installed in the vicinity of an enginewhich is continuously subjected to large vibrations. Accordingly, it isvery important in practical use that they have large compressionstrength on the outer peripheral side thereof so that they are notbroken even if they are installed in the case while being gripped with arelatively large force.

Further, in a honeycomb structure used as, for example, an exhaust gaspurifying filter, a filter regenerating process is executed therein tocombust and remove carbon fine particles accumulated therein. In thefilter regenerating process, the honeycomb structure is locally exposedto a high temperature. At the time, a problem arises in that cracks andthe like are liable to occur in the honeycomb structure in a relativelyshort time of use because the thermal stress of the honeycomb structureis locally increased due to a difference of thermal expansion amongrespective portions thereof.

Further, a honeycomb structure used as a catalyst, a filter and the likefor purifying exhaust gases is required to further improve a purifyingcapability by more reducing a thermal capacity and a pressure loss,improving a trapping efficiency, and the like. Thus, a decrease inthickness of the partition walls of the honeycomb structure and anincrease in the porosity thereof are in progress in correspondence tothe above requirement.

Incidentally, there has been proposed a honeycomb structure that is madeby joining a plurality of honeycomb segments in order to be formed in adesired shape through joint members as a countermeasure against theproblem of occurrence of cracks and the like in the honeycomb structurewhich are caused by being locally exsposed to high temperature (refer toJapanese Patent Publication (KOKOKU) No. 61-51240 and Japanese PatentApplication Laid-Open No. 8-28246).

Further, in a honeycomb structure made integrally in its entirety, therehas been proposed a structure, in which the partition walls of an outerperipheral portion are made thicker than internal partition walls, orthe porosity of the partition walls thereof is reduced to providesufficient compression strength even when the outer periphery of thehoneycomb structure is gripped with a large force (refer to JapanesePatent Application Laid-Open Nos. 54-150406 and 55-147154).

However, any countermeasure is not taken into consideration to preventbreakages which selectively occur in particular portions of theso-called segment type honeycomb structure composed of a plurality ofsegments joined to each other when the honeycomb structure is used in acase by being gripped therein. Accordingly, when the honeycomb structureis mounted, a sufficient performance cannot be necessarily obtained.Further, in the honeycomb structure made integrally as a whole, theentire partition walls of the outer peripheral portion thereof are madethicker than inner partition walls and the porosity and the like thereofare reduced. With this arrangement, a thermal capacity and a pressureloss are greatly increased or a trapping efficiency is greatly reduced.Accordingly, a high purification performance, which is required by thetimes, cannot be achieved while satisfying desired compression strength.

Further, in the honeycomb structure made integrally as a whole, when ahoneycomb structure in which partition walls are thicker in an outerperipheral portion than in an inner portion is made, and when ahoneycomb structure in which cell density is larger in an outerperipheral portion than an inner portion is made, a difference ofextrusion resistance is increased among the respective portions of thehoneycomb structure by greatly changing a partition wall thickness and acell density, from which a problem in extrusion forming arises in thatdefective forming is liable to occur in a resultant honeycomb structure.Likewise, when a honeycomb structure and the like, in which the porosityof the partition walls of an outer peripheral portion is smaller thanthat of the partition walls of an inner portion, is made, a materialused in the partition walls of the outer peripheral portion isordinarily replaced by a material having a smaller porosity. In themethod, however, it is very difficult to provide the porosity of thepartition walls with a desired change.

DISCLOSURE OF THE INVENTION

An object of the present invention, which was made in view of theproblems described above, is to provide a honeycomb structure which canhighly suppress occurrence of cracks and the like when it is used andcan exhibit performances such as a high purification performance and thelike while maintaining sufficient compression strength in practical use.

As a result of diligent studies for solving the problems describedabove, the inventors have found that the problems such as the defectiveforming and the like of the conventional honeycomb structure madeintegrally as a whole can be solved in a way that the outer periphery ofthe honeycomb structure is composed of honeycomb segments having largecompression strength by changing the structure of each segment thatcontributes to the compression strength such as a partition wallthickness, partition wall density, cell density, cell shape or the like.Further, as a result of repeated studies, the inventors have found thatwhen a honeycomb structure, which is formed by joining a plurality ofhoneycomb segments, is used in a case by being gripped therein, thestress applied on one honeycomb segment by gripping the honeycombstructure is eased only by the deformation of the partition wallsexisting in the honeycomb segment due to the existence of a joint membersuch as cement and the like, and thus a difference of compressionstrength is liable to occur in the respective honeycomb segments fromthe structural characteristics thereof, as compared with the honeycombstructure made integrally as a whole in which the stress can be eased bythe deformation of the partition walls of the honeycomb structure in itsentirety. In addition, the inventors have found from the furtherinspection of broken portions that the breakages selectively occur inthe honeycomb segments in which the partition walls in the partitionwall length direction on a cross section perpendicular to the fluidpassage direction of a cell form an angle of 20 to 70° with respect to atangent to the outer periphery of the honeycomb structure at thepositions where the respective partition walls contact with an outerperipheral wall, and thus the above problems can be solved by applying astructure for increasing the compression strength only to thesehoneycomb segments. Accordingly, the inventors have completed thepresent invention.

That is, according to the present invention, there is provided ahoneycomb structure made by joining a plurality of honeycomb segments ineach of which a plurality of cells each having a quadrilateral sectionalshape (which means a sectional shape in a direction perpendicular to thefluid passage direction of a cell) are formed by being defined by aplurality of partition walls which are at right angles to each other,characterized in that at least some of honeycomb segments constitutingat least a portion of the outer periphery of the honeycomb structurehave a structure in which the compression strength is larger than thatof the honeycomb segments constituting the other portions of thehoneycomb structure.

Here, the term “compression strength” used in the description meanspressure by which the respective honeycomb segments are broken when thehoneycomb structure formed by joining the respective honeycomb segmentsis pressurized by static water pressure. The compression strength usedhere is mainly pressure in a direction perpendicular to the fluidpassage direction of a cell.

In the present invention, of the honeycomb segments constituting theouter periphery of the honeycomb structure, it is preferable thathoneycomb segments in which the partition walls in the partition walllength direction on a cross section perpendicular to the fluid passagedirection of a cell (hereinafter, may be simply referred to as“partition wall length direction”) form an angle of 20 to 700 withrespect to a tangent to the outer periphery of the honeycomb structureat the positions where the respective partition walls contact with anouter peripheral wall, have a structure in which the compressionstrength is larger than that of the honeycomb segments constituting theother portions of the honeycomb structure.

More specifically, of the honeycomb segments constituting the outerperiphery of the honeycomb structure, it is preferable that honeycombsegments in which the partition walls in the partition wall lengthdirection form an angle of 20 to 70° with respect to a tangent to theouter periphery of the honeycomb structure at the positions where therespective partition walls contact with an outer peripheral wall, arecomposed of partition walls having an average partition wall thicknesslarger than the honeycomb segments constituting the other portions ofthe honeycomb structure, having a porosity smaller than the honeycombsegments constituting the other portions of the honeycomb structure, orhaving a cell density larger than the honeycomb segments constitutingthe other portions of the honeycomb structure.

Further, in the present invention, of the honeycomb segmentsconstituting the outer periphery of the honeycomb structure, it is alsopreferable that honeycomb segments in which the partition walls in thepartition wall length direction form an angle of 20 to 70° with respectto a tangent to the outer periphery of the honeycomb structure at thepositions where the respective partition walls contact with an outerperipheral wall, further include partition walls connecting the opposingcorners of the respective cells each having a rectangular sectionalshape, and cells each having a triangular sectional shape in a radialdirection are formed between the respective partition walls.

Further, according to the present invention, a honeycomb structure ismade by joining a plurality of honeycomb segments in each of which aplurality of cells each having a rectangular sectional shape in a radialdirection are formed between the partition walls which are at rightangles to each other, characterized in that the honeycomb segments havea structure in which all the partition walls in the partition walllength direction on a cross section perpendicular to the fluid passagedirection of a cell form an angle of 0° or more to less than 200, ormore than 70° to 90° or less with respect to a tangent to the outerperiphery of the honeycomb structure at the positions where therespective partition walls contact with an outer peripheral wall.

As described above, in the present invention, the structure, whichrelates to the compression strength such as the partition wallthickness, the partition wall density, the cell density, and the like,is changed in the unit of the honeycomb segments constituting thehoneycomb structure in stead of simply composing the outer periphery ofthe honeycomb structure having large compression strength. Accordingly,the respective portions of the honeycomb structure can be easilycomposed of a different material, thereby the density of the partitionwalls can be greatly changed. Likewise, in the honeycomb structure, thepartition walls are not deformed at all even if the partition wallthickness and the cell density are greatly changed.

Further, in the present invention, the partition wall thickness, thepartition wall density, the cell density, and the like are increased inonly the honeycomb segments disposed at the particular positions, thatis, “the honeycomb segments in which the partition walls in thepartition wall length direction form an angle of 20 to 70° with respectto a predetermined tangent to the outer periphery of the honeycombstructure”. Accordingly, performances such as a high purificationperformance and the like can be exhibited while maintaining desiredcompression strength. In particular, since it is important to secure thecompression strength in the honeycomb structure having a thin wallthickness and a high porosity, the present invention is useful in thatthe compression strength, which is required in practical use, can besecured while satisfying a requirement for the improvement ofperformance by decreasing a wall thickness and increasing a porosity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is an upper surface view schematically showing an embodimentof a honeycomb structure according to the present invention as well asFIGS. 1(b) and 1(c) are enlarged views of a part of the honeycombstructure.

FIG. 2 is an enlarged view of a part of the honeycomb structureaccording to the present invention for schematically showing apositional relationship between a partition wall length direction on aradial cross section of the honeycomb structure in the partition wallsof each honeycomb segment and an outer peripheral tangential line of thehoneycomb structure.

FIG. 3 is a graph showing a result of compression strength between ahoneycomb structure of an example 1 and a honeycomb structure of acomparative example 1.

Note that, in the above figures, the following reference numerals denotemembers or structures as shown below.

EXPLANATION OF REFERENCE NUMERALS

Reference numerals 1, 1 a, 1 b and 2 denote honeycomb segments,respectively, 3, 3 a, 3 b denote partition walls, respectively, 4denotes a cell, 5 denotes the outer periphery (wall), 10 denotes ahoneycomb structure, and 21 denotes a joint member.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be specifically explainedbelow based on the figures. FIG. 1(a) is an upper surface viewschematically showing an embodiment of a honeycomb structure of thepresent invention, FIGS. 1(b) and 1(c) are enlarged views each showing apart of the honeycomb segment at a particular position in the honeycombstructure, and FIG. 2 is an enlarged view of a part of the honeycombstructure according to the present invention for schematically showing apositional relationship between a partition wall length direction inrespective honeycomb segments and an outer peripheral tangential line ofthe honeycomb structure.

As shown in FIG. 1, the honeycomb structure 10 of the present inventionis composed of a plurality of joined honeycomb segments 1, 2 each havinga plurality of cells 4 between a plurality of partition walls 3, eachcells 4 having a rectangular cross section. In the honeycomb structure10, at least portions 1 a of the honeycomb segments 1 constituting theouter periphery 5 of the honeycomb structure 10 have a structure inwhich the compression strength is larger than that of the honeycombsegments 2 constituting the other portions of the honeycomb structure10.

The honeycomb structure 10 will be specifically explained below.

The honeycomb structure 10 of the present invention is made by joining aplurality of honeycomb segments which are different in at least aphysical property, for example, the segments 1 mainly disposed in aperipheral portion and the segments 2 mainly disposed in a centralportion. Accordingly, even if the honeycomb structure is composed of amaterial having a large coefficient of thermal expansion such as SiC andthe like, occurrence of cracks and the like can be prevented by reducinggeneration of local thermal stress in use.

Further, in the respective honeycomb segments constituting the honeycombstructure according to the present invention, the plurality of cells 4being defined between the respective partition walls 3 which are atright angles to each other are formed in a quadrilateral sectionalshape, that is in a square sectional shape or a rectangular sectionalshape. Accordingly, not only a die for extrusion forming of therespective segments can be extremely easily made but also the heat shockresistance of a resultant honeycomb structure can be also improved.

Further, in the honeycomb structure 10 of the present invention, thehoneycomb segments 1 a, which is a kind of a series of the segments 1disposed to constitute the outer periphery 5 of the honeycomb structure10, have a structure in which the compression strength is larger thanthat of the honeycomb segments 2 that are segments other than thesegments used to constitute the outer periphery 5 of the honeycombstructure 10 (hereinafter, the above structure may be referred to as“high strength structure”). With the above arrangement, a honeycombstructure having desired compression strength can be made withoutoccurrence of the deformation and the like of the partition walls due toa difference of extrusion resistance as in the manufacture of anintegral type honeycomb structure having partition walls of differentthickness in each position thereof.

As shown in FIG. 2, as the honeycomb segments of high strength structureto be disposed in a portion of the outer peripheral wall 5 at theposition C where the partition walls length direction X on a crosssection perpendicular to the fluid passage direction of a cell 4 form anangle of 20 to 70° with respect to a tangent T to the outer periphery ofthe honeycomb structure 10, it is preferable to dispose the honeycombsegments (which may be simply referred to as “low strength honeycombsegments” hereinafter) 1 a in which partition walls 3 a form theabove-mentioned angle with respect to a tangent to the outer periphery.It is more preferable to dispose the honeycomb segments 1 a in which thepartition walls 3 a in the partition wall length direction X form anangle of 30 to 60° with respect to a tangent T to the outer periphery,and it is particularly preferable to dispose the honeycomb segments 1 ain which the partition walls 3 a in the partition wall length directionX form an angle of 40 to 50° with respect to a tangent T to the outerperiphery.

In the honeycomb structure composed of the honeycomb segments joinedtogether, the honeycomb segments 1 a in which the partition walls 3 a inthe partition wall length direction X on the cross section perpendicularto the fluid passage direction of a cell is within the above range, havevery small compression strength. Accordingly, the above positions areintensively reinforced using the honeycomb segments 1 a so that anincrease in a thermal capacity and a pressure loss and a decrease in atrapping efficiency can be suppressed as much as possible whileeffectively increasing the compression strength. As a result, it ispossible to exhibit a high purification performance in response torecent requirements which becomes increasingly severe. That is, sincethe segments 1 a are compressed from an oblique direction (from aso-called C-axis) in canning, they are located at portions which arebroken first when a compression load is applied to the overall honeycombstructure. Accordingly, an advantageous effect of increasing strength isexhibited by intensively reinforcing the segments 1 a while minimizingother effects.

Of the plurality of the honeycomb segments 1 which constitute the outerperiphery of the honeycomb structure 10, honeycomb segments having alarge partition wall thickness, a small average partition wall porosityor a large cell density as compared with honeycomb segments 1 b and thesegments 2, which constitute the other portions of the honeycombstructure, may be used and disposed as the above low strength honeycombsegments 1 a.

In the present invention, the low strength honeycomb segments 1 a as thehigh strength structure may be composed only one kind of the segmentshaving the above properties. However, it is preferable to providesegments composed of a combination of at least two kinds of these highstrength structures from the standpoint that the compression strengthcan be secured without deteriorating the properties such as thepurification performance and the like by executing the intensivereinforcement as described above.

In the present invention, the partition wall thicknesses of thehoneycomb segments 1, 2 are not particularly limited. However, it ispreferable that the ratio of the thickness of the partition wall 3 a ofthe low strength honeycomb segments 1 a to the thickness of thepartition walls of the honeycomb segments 1 b and 2 other than thehoneycomb segments 1 a (ratio of average partition wall thickness of thelow strength honeycomb segments/average partition wall thickness ofbasic honeycomb segments) is preferably in the range of 1.1 to 5.0, morepreferably in the range of 1.2 to 3.0, and particularly preferably inthe range of 1.2 to 2.0 from the standpoint of preventing the localbreakage of the partition walls while exhibiting a desired purificationperformance by suppressing the increase in the pressure loss, thethermal capacity, and the like.

Further, to exhibit the desired purification performance by suppressingthe increase in the pressure loss, the thermal capacity, and the like,the maximum thickness of the partition walls 3 of the respectivehoneycomb segments including the honeycomb segments 1 a is preferably inthe range of 0.03 to 1.3 mm, more preferably in the range of 0.05 to 0.8mm, and particularly preferably in the range of 0.1 to 0.5 mm as long asthe above relationship is established.

In the present invention, the average porosity of the partition walls 3that constitute the respective honeycomb segments is not particularlylimited. However, the average porosity of the partition walls 3 aconstituting the low strength honeycomb segments 1 a is made smallerthan that of the partition walls 3 b constituting the basic honeycombsegments 1 b and 2 preferably by 5 to 90%, more preferably by 10 to 70%,and particularly preferably by 10 to 45% from the standpoint ofpreventing the local breakage of the partition walls while exhibitingthe desired purification performance by suppressing the increase in thethermal capacity and the decrease in the trapping efficiency.

Further, in the partition walls constituting any of the honeycombsegments, the average porosity of the partition walls is set topreferably 5 to 80%, more preferably to 10 to 70%, and particularlypreferably to 20 to 65%, as long as the above relationship isestablished, to secure the pressure loss and desired mechanicalstrength.

In the present invention, the cell density of the respective honeycombsegments is not particularly limited. However, the cell density of thelow strength honeycomb segments 1 a is made larger than that of thebasic honeycomb segments 1 b and 2 preferably by 10 to 400%, morepreferably by 15 to 200%, and particularly preferably by 20 to 100% toprevent the local breakage of the partition walls while preventing theincrease in the pressure loss.

In any of the honeycomb segments 1, 2, the cell density is preferably inthe range of 100 to 1500 cells/in² (15.5 to 232.5 cells/cm²), morepreferably in the range of 150 to 1200 cells/in² (23.3 to 186.0cells/cm²), and particularly preferably in the range of 200 to 90°cells/in² (31.0 to 139.5 cells/cm²), as long as the above relationshipis established, to secure the pressure loss and the desired mechanicalstrength.

Note that, in the present invention, since the above high strengthstructure is provided in the unit of segment, a honeycomb structurehaving the high strength structure can be made without deforming thepartition walls in forming and the like.

More specifically, the compression strength of a thin wall honeycombstructure having a partition wall thickness of 0.1 mm or less and thecompression strength of a honeycomb structure having a high porosity of50% or more can be effectively improved by setting the ratio of theaverage partition wall thickness of the low strength honeycomb segments1 a to that of the basic honeycomb segments 1 b, 2 to 1.4 to 2.0; makingthe porosity of the partition walls constituting the low strengthhoneycomb segments 1 a smaller by 3 to 45% than that constituting thebasic honeycomb segments 1 b, 2; and making the cell density of the lowstrength honeycomb segments larger by 20 to 100% than that of the basichoneycomb segments. In addition, a high performance achieved by reducingthe wall thickness and increasing the porosity is secured.

Exemplified further as the high strength structure of the presentinvention is a high strength structure which is arranged such that thelow strength honeycomb segments 1 a are further provided with partitionwalls for connecting the opposing corners of cells each having a squaresectional shape so that cells each having a triangular sectional shapeare formed between the respective partition walls.

In the present invention, the ratio of the cells having the triangularsectional shape to all the cells in the low strength honeycomb segments1 a may be set within an appropriate range in consideration of the otherstructures relating to the compression strength such as the partitionwall thickness, the porosity, and the like, conditions under which thesegments 1 a are used, and the like. However, in the low strengthhoneycomb segments 1 a, the ratio of the cells having the triangularsectional shape to all the cells is preferably set to 10 to 70%, andmore preferably to 30 to 50% to effectively improve the compressionstrength while suppressing the increase in the pressure loss.

Further, of the honeycomb segments 1 constituting the outer periphery ofthe honeycomb structure 10, the honeycomb segments 1 a, in which thepartition walls 3 a in the partition wall length direction X on thecross section perpendicular to the fluid passage direction of a cellform an angle of the above range with respect to a tangent T to theouter periphery of the honeycomb structure 10 at the position C wherethe respective partition walls contact with the outer peripheral wall 5as shown in FIG. 2, is replaced by the honeycomb segments as the highstrength structure in the present invention in which the partition wallshave an angle of the range other than the above range with respect to atangent T. That is, the outer periphery 5 of the honeycomb structure 10is composed of honeycomb segments in which all the partition walls inthe partition wall length direction X on a cross section perpendicularto the fluid passage direction of a cell form an angle of o 0° or moreto less than 200, or more than 70° to 90° or less with respect to atangent T to the outer periphery of the honeycomb structure 10 at theposition C where the respective partition walls contact with the outerperipheral wall 5.

In the high strength structure arranged as described above, the desiredcompression strength can be obtained without changing any of thepartition wall thickness, the partition wall porosity, and the likedescribed above. Accordingly, a higher purification performance and thelike can be exhibited because the increase in the pressure loss and thethermal capacity, and the like can be suppressed. Note that in thepresent invention, the partition wall length direction X can be adjustedfor each of the honeycomb segments 1, 2. Thus, it is not necessary toproduce a die having a complex shape and the partition walls are notdeformed by the difference of extrusion resistance in forming, differentfrom the integrally formed honeycomb structure.

In the honeycomb segments 1 constituting the outer periphery of thehoneycomb structure 10 according to the present invention, all thepartition walls in the partition wall length direction X form preferablyan angle of 0° or more to less than 20° or less, or more than 700 to 90°or less, and more preferably an angle of 0° or more to less than 150, ormore than 750 to 90° or less, with respect to a tangent to the outerperiphery at the positions C where the respective partition wallscontact with the outer peripheral wall in order to increase thecompression strength effectively.

Note that principal high strength structures of the present inventionhave been described above. However, it is needless to say that thehoneycomb structure of the present invention may be provided with anyother high strength structures as long as they have a structure forincreasing the compression strength. That is, the object of the presentinvention can be achieved by improving the strength of a material by,for example, coating or dipping a catalyst and the like carried byalumina and the like.

In the present invention, exemplified as the material of the honeycombsegments 1, 2 is ceramics whose main crystalline phases are composed ofat least one kind selected from the group consisting of, for example,non-oriented cordierite, oriented cordierite, mullite-zircon, mullite,zircon, cordierite-mullite, mullite-aluminum titanate, clay bond siliconcarbide, silicon carbide, metal silicon, zirconia-spinel, etc. Amongthem, it is preferable to use the silicon carbide or the metal siliconand the silicon carbide as main crystals because they are excellent inheat resistance and heat conductivity and can reduce the thermal stressof the respective portions of a filter while securing the high heatconductivity of the overall filter.

In the present invention, the number of the honeycomb segments 1, 2constituting the honeycomb structure 10 is not particularly limited.However, it is preferable to compose the honeycomb structure 10 havingnine or more honeycomb segments 1, 2 to reduce the occurrence of localthermal stress. Further, it is preferable to compose the outer periphery5 of the honeycomb structure 10 having eight or more honeycomb segments1 and it is more preferable to compose the same having twelve or morehoneycomb segments 1 in order to intensively improve the strength of theportions which are liable to be broken when they are gripped to therebyprevent the deterioration of the basic performances such thepurification performance and the like.

Further, it is sufficient for the cells 4 of the present invention tohave any of quadrilateral sectional shapes, and a square sectionalshape, a rectangular sectional shape, and the like can be exemplified asthe sectional shapes. However, it is preferable that the cells 4 areformed in the rectangular sectional shape in a radial direction to moreimprove the heat shock resistance of the honeycomb structure 10. Incontrast, when the cells are formed in the square sectional shape in theradial direction, the compression strength of the honeycomb structure 10can be improved. Accordingly, it is also preferable that at least someof the honeycomb segments 1 constituting the outer periphery 5 of thehoneycomb structure 10 have cells formed in the square sectional shapein the radial direction.

Note that when the honeycomb structure of the present invention is usedas an exhaust gas purification filter, it is sufficient for theplurality of cells 4 of the respective honeycomb segments 1, 2 to havesuch a structure that the cells are alternately plugged with a plugmember at both the ends thereof where the cells 4 open.

Further, when the honeycomb structure 10 of the present invention isused as a catalyst, it is sufficient to cause the partition walls 3 tocarry a metal having a catalytic property, for example, Pt, Pd, Rh, etc.

Although the honeycomb structure 10 of the present invention isintegrated by joining the honeycomb segments 1, 2 described above, thejoint structure, a kind of a joint agent, and the like are notparticularly limited. For example, adjacent honeycomb segments 1 a, 1 bmay be joined together with a joint agent 21 through a part ofconfronting sides or through the overall confronting sides (FIG. 1(a)shows an example that the honeycomb segments are joined through theoverall sides). It is preferable that the joint agent 21 be mainlycomposed of a heat resistant inorganic compound because it is excellentin heat resistance and the heat shock resistance. Further, it ispreferable that the heat resistant inorganic compound contains at leastone kind of, for example, a fiber material containing ceramics similarto that of the honeycomb segments 1, 2 described above and metal, andcement such as hydrated cement, thermosetting cement, etc.

Note that the respective segments that constitutes the honeycombstructure 10 of the present invention are made by a method ordinarilyexecuted in a ceramic honeycomb structure and they are joined togetherby a method used to join ceramic structures. For example, the honeycombstructure 10 can be obtained as described below. That is, a clay isobtained by kneading a raw material, which is composed of a desiredceramic material added with a binder, a dispersing agent and the like,and a desired amount of water; the clay is formed to honeycomb segmentshaving a desired shape by extrusion forming and the like and dried orfurther fired, and then the respective honeycomb segments are jointedtogether.

Although the present invention will be explained in more detail based onspecific examples, it is by no means limited to these examples.

EXAMPLE 1

Mixed powder containing 75% by weight of SiC powder and 25% by weight ofmetal Si powder was prepared as a ceramic material. 6 parts by weight abinder, which was composed of methyl cellulose and hydroxypropoxylmethyl cellulose, 0.8 part by weight of a surface-active agent, and 22parts by weight of water were added to 100 parts by weight of theresultant blended powder. The resultant was kneaded in a kneadingmachine, thereby a clay having plasticity was obtained.

Next, the clay was subjected to extrusion forming to form the honeycombsegments 1 a, 1 b, and 2 having shapes corresponding to the shapes ofthe respective pieces of 16 portions obtained by dividing the honeycombstructure 10 having an cylindrical outside appearance as shown in FIGS.1(a) and 2. At the time, in all the four honeycomb segments 1 a in whichthe partition walls in the partition wall length direction X on a crosssection perpendicular to the fluid passage direction of a cell formed anangle of 20 to 70° with respect to a tangent T to the outer periphery atthe positions where the respective partition walls 3 contacted with anouter peripheral wall (the partition walls 3 constituted the outerperipheral wall of the honeycomb structure after they were connectedthereto), the thickness of the partition walls of was set to 0.432 mmand the outside dimension of each honeycomb segment was set to 35 mm×35mm×154 mm,. In contrast, the thickness of all the partition walls of theother honeycomb segments 1 b, 2 was set to 0.304 mm. Further, all thehoneycomb segments 1, 2 had cells 4 each having a square sectionalshape, and a cell density was set to 300 cells/in² (46.5 cells/cm²).

Next, after the respective honeycomb segments were microwave or hot-airdried, a joint agent composed of cement was applied to the sides of therespective honeycomb segments so that the honeycomb segments wereintegrated. Further, the integrated honeycomb segments were fired atabout 1550° C. in an Ar atmosphere, thereby a honeycomb structure of 143mmø×154 mmL was made. Note that partition walls constituting therespective honeycomb segments had approximately the same porosity ofabout 52%.

COMPARATIVE EXAMPLE 1

A honeycomb structure was made similarly to the example 1 except that inthe four honeycomb segments 1 a, in which the partition walls in thepartition wall length direction X on a cross section perpendicular tothe fluid passage direction of a cell formed an angle of 20 to 700 withrespect to a tangent T to the outer periphery at the positions where therespective partition walls 3 contacted with an outer peripheral wall 5(the partition walls 3 constituted the outer peripheral wall of thehoneycomb structure after they were connected thereto) as shown in FIGS.1, 2 in the example 1, the thickness of the partition walls was set to0.304 mm.

(Evaluation Method)

Five honeycomb structures were made, respectively by the manufacturingmethods described in the example 1 and the comparative example 1, andthe compression strength thereof was evaluated as shown below.

First, both the ends of the respective honeycomb structures were coveredwith metal sheets having the same diameter as the respective honeycombstructure. Further, after the metal sheets were fixed by rubber tubeshaving the same diameter as the respective honeycomb structure, thehoneycomb structures were hermetically sealed by bonding rubber tapes tothe rubber tubes and their vicinities so that water does not come intothe honeycomb structures.

Next, in this state, the honeycomb structures were dipped into water,water pressure was increased until any arbitrary portion of eachhoneycomb structure was broken, and water pressure when the breakageoccurred was evaluated as compression strength (MPa).

(Result of Evaluation)

As shown in FIG. 3, in the honeycomb structures of the comparativeexample 1 in which all the partition walls had the same thickness, thecompression strength was 2 MPa. Whereas, in the honeycomb structures ofthe example 1 in which the thickness of the partition walls of only theparticular honeycomb segments was set from 0.304 mm to 0.432 mm, thecompression strength was greatly increased to about 4 MPa. Further, asto a mode of breakage, in the honeycomb structures of the comparativeexample 1, the partition walls of the honeycomb segments 1 a shown inFIG. 1(a) were selectively broken. Whereas, in the honeycomb structuresof the example 1, any pattern was not particularly found as to brokenportions. Accordingly, it was found that the strength of the honeycombstructure of the example 1 was significantly improved by intensivelyreinforcing the particular portions using the particular honeycombsegments. It should be noted that, in a honeycomb structure having thesame arrangement as the honeycomb structure of the embodiment 1, it canbe naturally expected that even if the thicknesses of all the partitionwalls are set to 0.432 mm, the compression strength which isapproximately the same as the honeycomb structure of the embodiment 1can be obtained and moreover a pressure loss and a thermal capacity aremuch smaller than those of the honeycomb structure of the comparativeexample.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, there can beprovided a honeycomb structure which can highly suppress occurrence ofcracks and the like when the honeycomb structure is used as well as canexhibit a high purification performance and the like while maintainingcompression strength that is sufficient in practical use.

1. A honeycomb structure made by joining a plurality of honeycomb segments in each of which a plurality of cells each having a quadrilateral sectional shape are formed by being defined by a plurality of partition walls which are at right angles to each other, characterized in that at least some of honeycomb segments constituting at least a portion of the outer periphery of the honeycomb structure have a structure in which compression strength is larger than that of the honeycomb segments constituting the other portions of the honeycomb structure.
 2. The honeycomb structure according to claim 1, wherein the honeycomb segments being disposed to at least a portion of the honeycomb segments constituting the outer periphery of the honeycomb structure and in which the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 20 to 70° with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall, have a structure in which the compression strength is larger than that of the honeycomb segments constituting the other portions of the honeycomb structure.
 3. The honeycomb structure according to claim 1, wherein the honeycomb segments being disposed to at least a portion of the honeycomb segments constituting the outer periphery of the honeycomb structure and in which the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 20 to 70° with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall, have a structure which comprises partition walls having an average partition wall thickness larger than the honeycomb segments constituting the other portions of the honeycomb structure.
 4. The honeycomb structure according to claim 1, wherein the honeycomb segments being disposed to at least a portion of the honeycomb segments constituting the outer periphery of the honeycomb structure and in which the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 20 to 70° with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall, have a structure which comprises partition walls having an average porosity smaller than the honeycomb segments constituting the other portions of the honeycomb structure.
 5. The honeycomb structure according to claim 1, wherein the honeycomb segments being disposed to at least a portion of the honeycomb segments constituting the outer periphery of the honeycomb structure and in which the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 20 to 70° with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall, have a cell density larger than the honeycomb segments constituting the other portions of the honeycomb structure.
 6. The honeycomb structure according to claim 1, wherein the honeycomb segments being disposed to at least a portion of the honeycomb segments constituting the outer periphery of the honeycomb structure and in which the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 20 to 70° with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall, further includes partition walls connecting the opposing corners of the respective cells each having a rectangular sectional shape, and cells each having a triangular sectional shape in a radial direction, which are formed between the respective partition walls.
 7. A honeycomb structure made by joining a plurality of honeycomb segments in each of which a plurality of cells each having a rectangular sectional shape in a radial direction are formed between partition walls which are at right angles to each other, characterized in that the outer periphery of the honeycomb structure is composed of honeycomb segments in which all the partition walls in the partition wall length direction on a cross section perpendicular to the fluid passage direction of a cell form an angle of 0° or more to less than 20°, or more than 70° to 90° or less with respect to a tangent to the outer periphery of the honeycomb structure at the positions where the respective partition walls contact with an outer peripheral wall. 